1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ion, a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium ion, a separator between the positive electrode and the negative electrode, and a nonaqueous electrolyte.
2. Description of the Related Art
For a battery to be used in portable electronic and communicating equipment such as a small-sized video camera, a mobile telephone and a notebook personal computer, recently, a nonaqueous electrolyte secondary battery represented by a lithium ion battery having an alloy or a carbon material capable of intercalating and deintercalating lithium ion as a negative electrode active material and lithium containing composite oxide, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) or lithium manganese oxide (LiMn2O4) as a positive electrode material has been put into practical use to be a battery having a small size, a light weight and a high capacity and capable of carrying out a charge and discharge.
Since lithium nickel oxide (LiNiO2) in the lithium containing composite oxide to be used for the positive electrode material of the nonaqueous electrolyte secondary battery has a feature of a high capacity and a drawback of a poor safety and a low discharge operating voltage, there is a problem in that it is inferior to the lithium cobalt oxide (LiCoO2). Moreover, lithium manganese oxide (LiMn2O4) has a rich source and is inexpensive and excellent in safety, and has a drawback that an energy density is low and manganese itself is dissolved at a high temperature. Therefore, there is a problem in that it is inferior to the lithium cobalt oxide (LiCoO2). At the present time, accordingly, the use of the lithium cobalt oxide (LiCoO2) to be the lithium containing composite oxide has been a mainstream.
Recently, a novel positive electrode active material such as olivine type LiMPO4 (M═Fe, Co) or 5V class LiNi0.5Mn1.5O4 has been studied and attention has been paid to the same material to be a positive electrode active material for a next generation nonaqueous electrolyte secondary battery. However, the positive electrode active material has a high discharge operating voltage of 4 to 5 V which exceeds the withstand potential (decomposition potential) of an organic electrolyte used in the nonacqueous electrolyte secondary battery. For this reason, a deterioration in a cycle is increased with a charge and discharge. Therefore, it is necessary to optimize other battery components, for example, the organic electrolyte so that there is a problem in that a long time is taken to achieve practical use.
On the other hand, lithium—manganese composite oxide having a 3V class layer structure has been proposed. There is a problem in that the lithium—manganese composite oxide having the layer structure has a large discharge capacity, while a discharge operating voltage tends to be divided into two stages in a 4V region and a 3V region and a cycle is deteriorated greatly. Moreover, the discharge is mainly carried out in the 3V region.
Therefore, there is a problem in that it is hard to directly substitute the same composite oxide for the use of a nonaqueous electrolyte secondary battery using, as a positive electrode active material, lithium cobalt oxide utilizing a 4V region which is currently put into practical use.
Under the circumstances, there has been proposed lithium—nickel manganese composite oxide (LiNi0.5Mn0.5O2) having a layer structure. The lithium—nickel—manganese composite oxide (LiNi0.5Mn0.5O2) having the layer structure includes a plateau in a 4V region and a discharge capacity per unit mass is comparatively high, that is, 140 to 150 mAh/g, and thus has an excellent characteristic as a novel positive electrode active material and has thereby been considered to be hopeful as one of the positive electrode active materials for a novel nonacqueous electrolyte secondary battery.
However, a positive electrode active material (LiNi0.5Mn0.5O2) greatly takes over the characteristics of lithium containing composite oxide mainly containing nickel in that an initial charge/discharge efficiency is low, that is, 80 to 90%, a discharge operating voltage is slightly low as in lithium nickel oxide and a cycle characteristic is poorer than that of lithium cobalt oxide, and there is a problem in that it is necessary to improve the characteristics more greatly.
On the other hand, JP-A-2001-23617 has proposed a lithium secondary battery in which a part of LiMnO2 in lithium—manganese composite oxide (LiMnO2) having a 3V class layer structure is substituted for Al, Fe, Co, Ni, Mg or Cr to obtain LiXMnYM1-YO2 1.0) (O<X≦1.1, 0.5≦Y≦1.0) so that a high temperature characteristic is improved. In the lithium secondary battery proposed in the JP-A-2001-23617, there is a problem in that it is hard to directly substitute the same composite oxide for the use of the lithium secondary battery utilizing, as a positive electrode active material, lithium cobalt oxide using a 4V region because a discharge voltage of LiXMnYM1−YO2 to be used as a positive electrode active material is low.